1. Field of the Invention
Aspects of the present invention relate to a membrane electrode assembly for a fuel cell, a method of manufacturing the same, and a fuel cell including the membrane electrode assembly.
2. Description of the Related Art
Fuel cells are devices in which a fuel and oxygen react electrochemically to generate electricity. Compared to other fuel cells, polymer electrolyte membrane fuel cells (PEMFCs) have a low operating temperature, high efficiency, high current density, high power density, short starting time, and a rapid response speed in response to a load change. In addition, since the PEMFCs use a polymer membrane as an electrolyte, the PEMFCs do not corrode, there is no need for electrolyte adjustment, the design of the PEMFCs is simple, and thus, the manufacture of the PEMFCs is easy. The PEMFCs have lower volume and weight compared to phosphoric acid type fuel cells that operate in the same manner as the PEMFCs.
When compared with secondary batteries used as power sources for electric vehicles, the PEMFCs have an energy density of about 200 to several thousands Wh/kg, while secondary batteries have an energy density of about 200 Wh/kg or less. In addition, in terms of a charging time, lithium secondary batteries require a charging time of about three hours, whereas the PEMFCs require a fuel injection time of merely several seconds. Thus, research and development of PEMFCs have been actively conducted worldwide in order to use them as power sources for electric vehicles, mobile and emergency power supplies, power supplies for military applications, and the like.
A conventional PEMFC includes a membrane electrode assembly (MEA) including an electrolyte membrane, an electrode catalyst layer, and a fuel diffusion electrode for uniformly diffusing fuels.
In direct methanol fuel cells (DMFCs), generally, when methanol and oxygen are supplied as a fuel, a methanol oxidation reaction and an oxygen reduction reaction occur in a catalyst layer, and thus, electrons move and carbon dioxide and water are generated as byproducts.
In order to enhance the performance of the DMFCs, it is important that a gas diffusion layer (GDL) has a structure such that fuel, i.e., an aqueous methanol solution, is uniformly diffused into an electrode catalyst layer, resulting in the occurrence of an active reaction, and carbon dioxide gases as a byproduct generated by the reaction are satisfactorily exhausted to outside of the electrode. However, since the aqueous methanol solution is hydrophilic and the carbon dioxide gases are hydrophobic, it is very difficult to construct the GDL such that inflow of fuel and discharge of byproducts are efficiently performed.
The MPL (micro-porous layer) is formed to coat the slurries prepared by mixing a water-repellent polymer resin and carbon into the carbon paper, and then drying and heat-treating the resultant. The hydrophilic and hydrophobic properties of the MPL are adjusted by controlling the amounts of the water-repellent polymer resin and carbon or by adding a surfactant thereto.
In addition, as a method of reducing the contact resistance between the electrode catalyst layer and the GDL, a method of compressing an MEA at a certain pressure has been disclosed (US Patent Publication 2003/0134178 A1). However, there is still a need for improvement in order to achieve a satisfactory reaction efficiency of a fuel cell including such GDL. The structure and adhesion state of the GDL of a fuel cell is largely dependent of the supply and exhaust of fuel, and the states of the electrode catalyst layer related to proton ion conductivity.